In a long-distance optical amplifying transmission system, optical fibers for transmission are connected and repeated by optical amplifiers, for example, erbium-doped optical fiber amplifiers. Gains of an optical amplifier generally have strong dependency on wavelengths and show gain wavelength characteristics of a mountain-shaped curve in the bandwidth of 1,550 nm used for long-distance optical fiber transmission.
As a method for equalizing, namely flattening, these gain wavelength characteristics of an optical amplifier, usually optical filters, for example, etalons, are connected to the optical amplifier in series. The optical filters need to have hollow or M-shaped transmittance characteristics for obtaining the lowest transmittance against the peak of gain wavelength characteristics of the optical amplifier. In short, an optical filter getting the lowest transmittance at the peak wavelength of gains of an optical amplifier has to be prepared.
In gain wavelength characteristics of an optical amplifier, however, both of the gain wavelength characteristics curve and the peak wavelength generally exhibit irregular. In spite of the above-mentioned circumstance, conventional arts try to equalize gain wavelength characteristics of individual optical amplifiers by using optical filters. Therefore, optical filters to be connected in series had to be arranged and adjusted for individual optical amplifiers. The gain peak wavelength of an optical fiber amplifier and the transmittance bottom wavelength of optical filters must be precisely coinciding. Furthermore, the trough depth of transmittance of an optical filter has to be as close as possible to the peak height of gain of an optical fiber amplifier. These arrangements are very difficult and, in addition to that, gain wavelength characteristics of an optical fiber amplifier vary depending to the gains, output power, etc. demanding to redesign optical filters in accordance with the specifications and the usages.
Accordingly, it needed plenty of work to obtain flat gain wavelength characteristics in a certain wavelength width, for example, in a wavelength division multiplexing transmission system.